1. Field of the Invention
The present invention relates to an improved process for the preparation of alkanediol-diaminobenzoates, which are useful as curing agents for the production of polyurethane ureas.
2. Description of Related Art
Hard, impact resistant, polyurethane products are desirable in the marketplace. Good properties in the final product are dependent, inter alia, upon the use of a good curative in the manufacture of the resin.
Alkanediol-diaminobenzoates have been used as curatives for the manufacture of polyurethane ureas. These curatives have been shown to be successful replacements for the industry standard 4,4'-methylene bis(2-chloroaniline), MbOCA, which has been on the suspect carcinogens list by OSHA and other regulatory agencies.
U.S. Pat. No. 3,932,360 (Cerankowski et al.) discloses cured diamine polyurethane products prepared by combining an isocyanate terminated urethane prepolymer with a compound of the formula: ##STR1## wherein X is a 2-12 carbon alkylene or cycloalkylene group, which may be substituted or unsubstituted.
These compounds were prepared by reacting p-nitrobenzoyl chloride with a diol and reducing the thus-formed compound to the diamine. This process is problematic in that it generates hydrochloric acid or amine hydrochlorides, which are corrosive and present environmental and disposal problems.
U.S. Pat. No. 4,283,549 (Holm) discloses a method of producing alkanediol-diaminobenzoates that includes esterifying nitro-benzoic acid and certain diols in a melt and then dissolving the intermediate in a solvent sparingly soluble in water, such as, an aromatic hydrocarbon, an ether, or an ester, and reducing the nitro groups with hydrogen gas. The sparingly soluble solvent is preferably anisole. The excess acid is converted to the sodium salt and removed with water. This process eliminates the corrosive aspect of hydrochloric acid. But the process, in itself, is complex and requires leaching, separation of acid, and recovery steps for the acid and the anisole solvent.
U.S. Pat. No. 4,476,318 (Harada et al.) discloses a process for the preparation of 1,3-propanediol-bis(p-aminobenzoate) wherein the process comprises reacting a p-aminobenzoic acid alkali metal salt with dihalogenated propane in an aprotic polar solvent. According to the inventors, with this process, diesterification proceeds without causing any undesirable side reactions and, therefore, the desired 1,3-propanediol bis(p-aminobenzoate) can be obtained in high purity and high yield. However, this process involves the problem of disposing of a halide-containing waste stream. It, too, requires separation and solvent recovery steps to make it efficient.
Japanes kokai 61012653 A2 discloses the manufacture of aminobenzoic acids by the reduction of nitrobenzoic acids in a water-immiscible hydrocarbon solvent in the presence of sulfur or sulfur-containing compounds and Raney nickel or sulfur-poisoned Raney nickel.
Japanese Kokai 58059949 A2 discloses the preparation of 1,3-propanediol-bis(p-aminobenzoate) by heating Cl(CH.sub.2).sub.3 R (R.dbd.Cl, Br) with sodium or potassium p-aminobenzoate in an aprotic dipolar solvent, e.g., dimethyl formamide, dimethyl sulfoxide, or N-methylpyrrolidone. Thus, 0.15 mol of Cl(CH.sub.2).sub.3 Cl was heated with 0.3 mol sodium p-aminobenzoate in dimethyl sulfoxide at 100.degree. for three hours to give 91.5 percent 1,3-propanediol bis(p-aminobenzoate) of 97.4 percent purity as opposed to a 3 percent yield with ethylene glycol as solvent. Cl(CH.sub.2).sub.n Cl(n=2, 4, 5, 8), instead of Cl(CH.sub.2).sub.3 Cl, gave the corresponding alkylene bis(p-aminobenzoates) in much lower yield.
Japanese Kokai 57120555 A2 discloses the preparation of compounds of the is structure (H.sub.2 NC.sub.6 H.sub.4 COO).sub.2 X, where X=(substituted) alkylene, by esterification of the benzoic acid with a dihaloalkane in the presence of a quaternary ammonium salt.
German OLS 2902740 discloses the preparation of alkanediol-bis(aminobenzoic acid esters) by esterifying nitrobenzoates with diols HO(CH.sub.2).sub.n OH (n=2, 3, 5, 6) in the presence of an aromatic sulfonic acid catalyst and reducing the resulting nitro compounds.
The disclosures of the foregoing are incorporated herein by reference in their entirety.
In view e various disadvantages attendant the methods described above, it is clear that a need remains for the development of a new method for the preparation of alkanediol-di-aminobenzoates.